In an electron microscope, for example, of the general type disclosed in U.S. Pat. No. 3,745,341 and No. 4,596,934, the specimen to be inspected is placed on a circular specimen grid, and the grid is positioned within a cylindrical counterbore formed within a movable specimen holder such as shown in FIG. 2 of U.S. Pat. No. 4,596,934. As also shown in this patent, the specimen grid is retained within the counterbore by a C-shaped wire retaining spring or ring which snap-fits into a corresponding internal groove formed within the holder.
When the wire retaining spring or ring is being removed from the mating groove, it is common to use a pair of forceps for gripping the wire ring. However, if the forceps slip on the ring or the ring slips from the forceps, the specimen may be damaged. Furthermore, because of the small size of the wire ring, for example, about 1/8 inch in diameter, it is difficult to determine whether or not the wire ring is properly seated within the mating groove or in the forceps. If the ring is not properly seated, the ring may become dislodged and lost within the microscope column while the holder is being manipulated for orientating the specimen grid. When the ring is lost, it can cause astigmatism, so that the microscope must be disassembled to find the lost ring and then reassembled, resulting in the microscope being out of service for several days.
In an attempt to avoid the above problems, another form of specimen grid retaining system has been used. This system incorporates an externally threaded ring which is threaded into mating threads formed within the specimen grid holder. However, it has been found difficult to obtain a precision retention force on the specimen grid with the use of the threaded ring. If an excessive torque is used on the threaded ring, the specimen grid may be damaged or fractured. In addition, it is not easy to insert the threaded ring and to obtain precision alignment and engagement of the internal and external threads.